Supercritical Water Gasification Process Research Articles

Technoeconomic analysis of supercritical water gasification of canola straw for hydrogen production

Production of hydrogen from renewable sources is gaining popularity to reduce our dependency on non-renewable fossil fuels to meet growing hydrogen demand. However, despite the great prospect of production of hydrogen from sustainable sources such as lignocellulosic biomass via supercritical water gasification (SCWG), it has not been commercialized at a large industrial scale. This is due to the lack of detailed economic analysis of SCWG of lignocellulosic biomass, owing to the complexity of the SCWG process and the heterogeneous nature of biomass. Therefore, to address this knowledge gap, in this study, a detailed technoeconomic analysis (TEA) of a conceptual SCWG pilot having the capacity to process 200 tons/day of canola straw for the production of green hydrogen was conducted. Mass and energy balance of conceptual pilot was performed using Aspen Plus ® simulation by utilizing experimental data and hydrogen yield of 41.62 mmol/g was obtained at optimized reaction conditions of 500 °C, 23 MPa, and 10 wt%. Economic analysis based on calculated mass and energy balance was performed using SuperPro software. Cash flow analysis for capital expenses (CAPEX) of 81 Million USD showed a high internal rate of return (IRR) of 38.9% and an undiscounted net present value (NPV) of 548 million USD. A minimum selling price (MSP) of 3.38 USD/kg H2 for produced hydrogen was estimated, which is lower than other renewable hydrogen production processes and comparable to non-renewable hydrogen production technologies. A high positive IRR and NPV, while a lower MSP showed that despite having a low technological readiness level (TRL) of 4, SCWG of lignocellulosic biomass is a technically feasible and economically viable process for the production of hydrogen. Furthermore, sensitivity analysis also revealed that capital expenses (CAPEX) and canola straw price had the highest influence on net present value (NPV) and MSP. However, overall NPV and MSP were highly stable to changes in parameters highlighting the robustness of the economic analysis.

Hydrogen production and elemental migration during supercritical water gasification of food waste digestate

The rapid growth of food waste (FW) is a huge challenge on a global scale, and anaerobic digestion is one of the most commonly used methods to deal with food waste, and the increasing amount of food waste digestate (FWD) produced by anaerobic digestion also poses a huge challenge to waste management. This paper explores supercritical water gasification (SCWG) as a valuable and innovative strategy for the conversion of FWD into H2-rich syngas. The research focuses on analyzing the effects of reaction temperature and residence time on syngas production, gas composition, element migration (C, N and P) during the SCWG process. Experimental results show that as the reaction temperature increases from 400 °C to 500 °C, the total syngas yield increases significantly, from 2.4 mol/kg to 9.7 mol/kg, especially the yields of H2, CO2, and CH4. As the reaction temperature increases and the residence time increases, the migration of carbon from the solid and liquid phases to the gas phase accelerates with increasing temperature and residence time, resulting in a higher proportion of carbon in the gas phase. In terms of liquid phase composition, nitrogenous compounds are significantly converted into ammonium (NH4+-N) at higher temperatures. In addition, the organic phosphorus is observed transferring into inorganic phosphorus, which are mainly apatite. This study explores the scalability of SCWG and its potential for the production of clean fuels, thereby contributing to the sustainable management of FWD.

A novel PEMFC-GT-ST power generation system employing supercritical water gasification of coal for enhanced hydrogen production

This study presents an innovative coal-fired power generation system that integrates a Proton Exchange Membrane Fuel Cell (PEMFC), Gas Turbine (GT), and Steam Turbine (ST) with a supercritical water gasification (SCWG) process designed to efficiently convert coal into hydrogen. A pivotal feature of this system is the first-time integration of advanced two-step SCWG technology with PEMFC, which significantly boosts hydrogen yield, thereby elevating overall efficiency. By effectively addressing the thermodynamic limitations of the Carnot cycle, the proposed system achieves an impressive coal-to-electricity conversion efficiency of 51.61%. Economic viability is demonstrated through a competitive levelized cost of energy (LCOE) of $0.052 per kWh and a rapid payback period of approximately 2.87 years. Furthermore, the study analyzes the impact of various operational parameters—such as coal feed capacity, water-to-coal ratio, operating temperatures of the PEMFC, fuel utilization rates, airflow, and hydrogen bypass flow—on system performance. The findings not only advance the efficiency of coal-to-electricity conversion but also contribute valuable insights to the evolution of sustainable coal-fueled power generation technologies.

Insight into particle size distribution and particle-scale reactions in a supercritical water fluidized bed reactor by CPFD method

Supercritical water gasification (SCWG) provides a promising solution for converting coal into hydrogen-rich gaseous. However, the modelling of the coal SCWG process still poses challenges due to the heterogeneous particle distribution, large quantity of particles, and complex chemical reactions. This study presents a novel three-dimensional numerical model based on computational particle fluid dynamics (CPFD) to study coal particle gasification in the reactor. The model integrates realistic coal particle size distribution, heat and mass transfer phenomena alongside heterogeneous/homogeneous chemical reactions across phases, elucidating multi-scale reacting flow behaviors and illustrating particle volume fraction, size, and velocity distributions. The obtained results reveal the evolution of coal composition, particle scale gasification process, and distribution of gaseous product components in the supercritical fluidized bed reactor. This work aims to serve as a reference for understanding the flow and heat transfer phenomena between supercritical water and coal particles in SCWG reactors.

Organic municipal solid waste derived hydrogen production through supercritical water gasification process configured with K2CO3/SiO2: Performance study

Cities worldwide face a significant public health and environmental challenge in handling municipal solid waste (MSW). This research exposed an effective utilization of MSW as the source for hydrogen production via a supercritical water gasification process under 450–650 °C at 15–45 min processing time. The impacts of gasification temperature and processing time on the functional properties of hydrogen production are studied. Its results are compared to identify the optimum processing temperature and processing time to adopt the system. Integrating 3 wt% silicon dioxide (SiO2) nanoparticles/3 wt% of potassium carbonate (K2CO3) enhances hydrogen production by increasing the catalyst's surface area and improving the stability of active sites, leading to more efficient gasification reactions. Increasing the gasification temperature from 450 to 650 °C significantly raises the hydrogen molar fraction and gas yield with peak gasification efficiency (GE) and hydrogen efficiency (HE) values. The gasifier functioned with catalyst (3 wt% K2CO3/SiO2) under 650 °C gasification temperature and 45min gasification time influenced better output responses like improved hydrogen gas yield of 63.7 mol/kg, higher gasification efficiency of 59.8 %, better hydrogen efficiency (63.4 %) and increased carbon conversion efficiency of 63.4 and 42.5 % respectively.

A Review of Catalyst Integration in Hydrothermal Gasification

Industrial scale-up of hydrothermal supercritical water gasification process requires catalytic integration to reduce the high operational temperatures and pressures to enhance controlled chemical reaction pathways, product yields, and overall process economics. There is greater literature disparity in consensus on what is the best catalyst and reactor design for hydrothermal gasification. This arises from the limited research on catalysis in continuous flow hydrothermal systems and rudimentary lab-scale experimentation on simple biomasses. This review summarizes the literature status of catalytic hydrothermal processing, especially for continuous gasification and in situ catalyst handling. The rationale for using low and high temperatures during catalytic hydrothermal processing is highlighted. The role of homogeneous and heterogeneous catalysts in hydrothermal gasification is presented. In addition, the rationale behind certain designs and component selection for catalytic investigations in continuous hydrothermal conversion is highlighted. Furthermore, the effect of different classes of catalysts on the reactor and reactions are elaborated. Overall, design and infrastructural challenges such as plugging, corrosion, agglomeration of the catalysts, catalyst metal leaching, and practical assessment of catalyst integration towards enhancement of process economics still present open questions. Therefore, strategies for catalytic configuration in continuous hydrothermal process must be evaluated on a system-by-system basis depending on the feedstock and experimental goals.

A comparative assessment of sewage sludge valorization via anaerobic digestion and supercritical water gasification: A techno-economic case study in Norway

This study compared the performance of supercritical water gasification (SCWG) to conventional anaerobic digestion (AD) for treating sewage sludge from a wastewater treatment plant (WWTP). Based on plant data from a WWTP in Norway, kinetic-based process models have been developed in Aspen Plus® to investigate the energy and economic feasibility of AD and SCWG for sewage sludge (SS) treatment. The findings showed that low ambient temperatures significantly reduced the energy efficiency of the AD, while their effect on the SCWG process was negligible. At the average ambient temperature of 7 °C and SS concentration of 7 wt%, AD and SCWG achieved thermal energy self-sufficiency (TES) of 0.76 and 0.63, respectively. Under these conditions, the AD and SCWG had energy conversion efficiencies (η) of 25.8% and 46.6%, respectively. However, increasing the SS content in the SCWG to 15 wt% improved the TES to 1.22 and η to 73.2%, making it a more attractive alternative to the AD.At 7 wt% SS concentration, the AD and SCWG processes required an investment cost of $ 4.08 million and $ 6.83 million, respectively. Under the study assumptions, neither AD nor SCWG proved profitable, revealing negative net present values (NPV) and operating costs of $0.96 million and $0.92 million, respectively.

Thermodynamic Model for Hydrogen Production from Rice Straw Supercritical Water Gasification.

Supercritical water gasification (SCWG) technology is highly promising for its ability to cleanly and efficiently convert biomass to hydrogen. This paper developed a model for the gasification of rice straw in supercritical water (SCW) to predict the direction and limit of the reaction based on the Gibbs free energy minimization principle. The equilibrium distribution of rice straw gasification products was analyzed under a wide range of parameters including temperatures of 400-1200 °C, pressures of 20-50 MPa, and rice straw concentrations of 5-40 wt%. Coke may not be produced due to the excellent properties of supercritical water under thermodynamic constraints. Higher temperatures, lower pressures, and biomass concentrations facilitated the movement of the chemical equilibrium towards hydrogen production. The hydrogen yield was 47.17 mol/kg at a temperature of 650 °C, a pressure of 25 MPa, and a rice straw concentration of 5 wt%. Meanwhile, there is an absorptive process in the rice straw SCWG process for high-calorific value hydrogen production. Energy self-sufficiency of the SCWG process can be maintained by adding small amounts of oxygen (ER < 0.2). This work would be of great value in guiding rice straw SCWG experiments.

Combining experiment and theory to study the mechanism of lignin supercritical water gasification

Supercritical water gasification (SCWG) presents notable advantages in biomass energy utilization. The exploration of the reaction mechanism of lignin in supercritical water holds significance in advancing and refining the biomass SCWG process. In this work, syringol was selected as a lignin model compound to study the SCWG mechanism of lignin through a combination of molecular simulation and experiments. The results show that the ether bond is rapidly cleaved in the reaction initial stages. While the phenolics dearomatization has a slower rate and temperature exerts a considerable influence on the dearomatization process. Four degradation pathways are proposed based on the molecular simulations. In the context of the dearomatization reaction, DFT calculations are performed to determine the energy barriers associated with each of the four pathways. Notably, the energy barrier for the most favorable pathway is found to be 291.96 kJ/mol. A lumped kinetic model is constructed based on the reaction pathway, and the reliability of the model is verified by both experiments and DFT calculations. This paper offers valuable assistance in optimizing reaction pathways.

Catalytic supercritical water gasification of oily sludge with the FeOOH/AC for hydrogen production

Along with the continuous increase of crude oil consumption in China, the production of oily sludge is increasing. The traditional oily sludge treatment technology cannot realize the purpose of green and efficient resource utilization of oily sludge. Supercritical water gasification hydrogen technology can efficiently convert oily sludge into hydrogen and realize the resource utilization of oily sludge. However, the traditional supercritical water gasification process requires high energy consumption. It is necessary to study the catalysts in the gasification process. Loaded catalysts have a large specific surface area and can effectively reduce the reaction activation energy. This paper investigated the effects of different catalyst types and concentrations on gasification. The results showed that adding FeOOH/AC effectively increased hydrogen production, and the best catalytic effect was achieved when the FeOOH loading was 10 wt%. The catalyst concentration was 5%, with a hydrogen production rate of 4.638 mol/kg. It was found that the activation energy of the reaction with the addition of the catalyst was reduced by 8.61 kJ/mol compared to that without the catalyst through kinetic analysis. 8.61 kJ/mol.

Prediction of Individual Gas Yields of Supercritical Water Gasification of Lignocellulosic Biomass by Machine Learning Models.

Supercritical water gasification (SCWG) of lignocellulosic biomass is a promising pathway for the production of hydrogen. However, SCWG is a complex thermochemical process, the modeling of which is challenging via conventional methodologies. Therefore, eight machine learning models (linear regression (LR), Gaussian process regression (GPR), artificial neural network (ANN), support vector machine (SVM), decision tree (DT), random forest (RF), extreme gradient boosting (XGB), and categorical boosting regressor (CatBoost)) with particle swarm optimization (PSO) and a genetic algorithm (GA) optimizer were developed and evaluated for prediction of H2, CO, CO2, and CH4 gas yields from SCWG of lignocellulosic biomass. A total of 12 input features of SCWG process conditions (temperature, time, concentration, pressure) and biomass properties (C, H, N, S, VM, moisture, ash, real feed) were utilized for the prediction of gas yields using 166 data points. Among machine learning models, boosting ensemble tree models such as XGB and CatBoost demonstrated the highest power for the prediction of gas yields. PSO-optimized XGB was the best performing model for H2 yield with a test R2 of 0.84 and PSO-optimized CatBoost was best for prediction of yields of CH4, CO, and CO2, with test R2 values of 0.83, 0.94, and 0.92, respectively. The effectiveness of the PSO optimizer in improving the prediction ability of the unoptimized machine learning model was higher compared to the GA optimizer for all gas yields. Feature analysis using Shapley additive explanation (SHAP) based on best performing models showed that (21.93%) temperature, (24.85%) C, (16.93%) ash, and (29.73%) C were the most dominant features for the prediction of H2, CH4, CO, and CO2 gas yields, respectively. Even though temperature was the most dominant feature, the cumulative feature importance of biomass characteristics variables (C, H, N, S, VM, moisture, ash, real feed) as a group was higher than that of the SCWG process condition variables (temperature, time, concentration, pressure) for the prediction of all gas yields. SHAP two-way analysis confirmed the strong interactive behavior of input features on the prediction of gas yields.

Experiment evaluation and thermodynamic analysis of pulping modification on ordered conversion of bark in supercritical water

Major obstacles of supercritical water gasification (SCWG) of lignocellulosic biomass, including ineffective pulping for energy self-sufficiency and a high conversion barrier significantly inhibited its large-scale development. In this study, the pulping effect of alkaline and salt pulping for high concentration bark slurry were compared systematically. The SCWG performances of bark (non-catalytic/catalytic) and bark slurry were evaluated. Based on energy self-sufficiency, a novel SCWG process of 50 wt% bark slurry was designated with a multistage reactor configuration and pulping process. The findings suggest that alkaline pulping had a wider applicability (reaching 50 wt%) and better rheological properties than salt pulping. Only 30 wt% bark slurry was capable for applying salt pulping because of solid residue with the excessive salt addition. Alkaline pulping resulted in an ordered conversion of 5 wt% and 10 wt% bark slurries during SCWG, with complete gasification of 5 wt% bark slurries prepared by KOH and NaOH at 650 and 680 °C, respectively. Besides, alkaline and salt pulping reduced the 26 % and 33 % carbon emissions, respectively. In the novel process, the optimal ratio of preheated water to bark slurry was 4 (water diversion to the primary and secondary gasification reactors of 1:3), which exhibits in energy and exergy efficiencies of 47.47 % and 48.63 %, respectively. This configuration reduces exergy losses in the gasification reactor and heat exchanger, aiding in the large-scale industrial output of SCWG.

Understanding the conversion mechanisms in supercritical water conditions of PET as a model compound of plastic wastes

In this work, polyethylene terephthalate (PET) was used as a model compound for understanding the conversion mechanism of plastic wastes through supercritical water gasification (SCWG) for producing: i) a gas rich in methane/hydrogen; and ii) for obtaining valuable molecules to be recycled in chemical processes (e.g., benzoic and acetic acids).The experiments were conducted at two different temperatures (above the critical point of water): 450 °C and 600 °C, in order to study the influence of this parameter on the conversion yield. The holding time in the reactor was set at 30 min for all trials. The experimental data were compared to the thermodynamic equilibrium calculations and the results showed that increasing the temperature from 450 °C to 600 °C, the total gas yield is highly improved (from 8.6 mol gas/kg of dried feedstock to 22 mol gas/kg of dried feedstock). The experiments with Raney-nickel catalyst indicated a significant improvement of the carbon conversion yield, which led to a higher gas production from mild to higher temperatures. The maximum total gas yield obtained with this catalyst was 63.4 mol gas/kg of dried feedstock at 600 °C, which is almost 3 times higher than that obtained at the same operating conditions without catalyst. The HHV of the obtained gas at 600 °C with the catalyst was 64.7 MJ/kg gas and 57.9 MJ/kg gas at 450 °C. Additionally at mild operating temperatures (450 °C), the obtained performance with the catalyst was even higher than that obtained at 600 °C without catalyst (50 mol gas/kg dried feedstock and 22 mol gas/kg dried feedstock, respectively), which is the major contribution of this work. At mild temperatures (450 °C) the methane production is highly favored, meanwhile at 600 °C the hydrogen production is enhanced. The methane yield obtained at 450 °C was 21.3 mol CH4/kg dried feedstock and the hydrogen yield obtained at 600 °C was 20.6 mol H2/kg dried feedstock. In the aqueous phases obtained from the experiments performed without catalyst at 450 °C, it was found valuable intermediate species such as benzoic and acetic acids (at a concentration of 4462 mg/L and 946 mg/L, respectively). This study represents a sustainable approach for valorizing plastic wastes by catalytic SCWG process.

Comparative Assessment of Hydrothermal Gasification and Anaerobic Digestion using Aspen Plus and SuperPro

Wet biomass is gaining increasing attention as an energy source globally. Various wet biomass materials like macroalgae, microalgae, sewage sludge, cattle manures, and food waste, are of high moisture contents, typically around 70% or more. There are two routes of high potential for converting these wet biomasss resoures to gaeous fuel, which are biomethanation and supercritical water gasification (SCWG). Biomethanation is conversion of wet-organic residues to biogas using via anaerobic digestion process. SCWG is a thermochemical conversion process, taking place in supercritical water producing enriched CH4 and H2 gases. In this study, the two processes are comperatively investigated using different process simulation softwares. Aspen Plus was employed for SCWG, while the biomethanation process was simulated with SuperPro. Various raw materials were used as input. We calculated the energy efficiency of the biomethanation process using experimental yields from the literature. For the SCWG process, we determined the higher heating value based on the simulated composition of the methane-enriched gas. Additionally, we conducted an economic analysis to compare the two processes, taking into consideration specific criteria relevant to the Norwegian context.

In-situ hydrogen production and battery electrode materials from metal effluent and biomass

Supercritical water gasification (SCWG) is an effective technique for the conversion of biomass into hydrogen-rich fuel gas. This work addresses two important aspects of the SCWG process by employing the concept of waste to wealth: first, the requirement of heterogeneous catalysts for better efficiency, and second, the safe disposal of byproducts of the process. Metal-contaminated aqueous effluent (Zn-EPW) is used as the reaction medium with the catalyst for single-step hydrothermal conversion of banana pseudo-stem (BPS) to synthesize multi-dimensional value-added products such as H2-rich fuel gas, nano metal–carbon hybrids (NCH), and treated aqueous phase. This study demonstrates the enhanced conversion of biomass into hydrogen-rich gas and recovery of heavy metals (HMs) in the form of nanometal carbon hybrids, which can be utilized to fabricate carbon-based electrodes in Li-S batteries. A comparative study of feedstock, such as a combination of Zn-EPW adsorbed on BPS (MA-BPS) with Millipore water, direct treatment Zn-EPW with BPS, and Millipore water with BPS (without HMs), was performed. The results inferred the catalytic effect of heavy metals increased the yield of hydrogen about ∼2.9 times with Zn-EPW and BPS and ∼1.7 times with MA-BPS and Millipore water over the non-catalytic operation. Furthermore, the obtained NCH were characterized to evaluate their performance for electrodes in energy storage devices. The electrochemical performance of NCH obtained after multi-step SCWG of MA-BPS with Millipore water and single-step treatment of Zn-EPW with BPS at 600 °C, have specific capacity of 610 and 615 mAh.g−1 at 0.1C respectively.

Thermodynamic and environmental analysis of an integrated multi-effect evaporation and organic wastewater supercritical water gasification system for hydrogen production

Organic wastewater poses significant environmental and human health risks. Supercritical water gasification (SCWG) presents a promising approach for converting organic waste into hydrogen-rich mixed gases. Nevertheless, the high moisture content present in organic wastewater hampers the energy efficiency of the SCWG process. To tackle this challenge, we have designed and developed an organic wastewater SCWG hydrogen production system incorporating a multi-effect evaporator. The developed system demonstrates the ability to achieve autothermal conditions for the treatment of swine wastewater with a solids concentration of 4.3%. Notably, this system exhibits impressive energy efficiency, reaching a maximum value of 44.88%, along with an exergy efficiency of 33.90% when processing 7% swine wastewater. Exergy analysis further reveals that the incorporation of a multi-effect evaporator leads to a reduction in exergy destruction within the gasification unit, oxidation unit, and heat exchanger. These improvements highlight the positive impact of the multi-effect evaporator on the overall performance of the supercritical water gasification system. Sensitivity analysis indicated that suitable concentrated concentration and an appropriate amount of preheated water can improve hydrogen production. The system minimum GWP reaches 5.27 kgCO2-eq/kgH2 after adding the CCUS. This work expands the range of applicable feedstocks for the autothermal SCWG process.

Supercritical water gasification of waste R410A refrigerant mixture for the resource utilization: ReaxFF reactive molecular dynamic simulation and density functional theory calculation study

Harmless treatment and resource utilization of the waste HFC refrigerants have gradually become the focus of attention. As a clean waste resource utilization technology, supercritical water gasification has good potential in converting waste refrigerant into high value-added chemicals and fuels. In this study, the supercritical water gasification mechanism of R410A, and the interaction mechanism between R125 and R32, and between H2O and R410A during the gasification process are investigated by using ReaxFF reactive molecular dynamic simulation and density functional theory calculation study. The results show that the main products obtained from the supercritical water gasification of R410A are HF, H2, CO and CO2. More than 90 % of F atoms in R410A can be mineralized to form HF molecules. H2 and CO are two important syngas products during the supercritical water gasification process. Hydrogen bonds are generated between R125 and H2O molecules and between R125 and R32 molecules, which leads to the decomposition of R125 molecules more easily, and the decomposition of R32 molecules more difficult. This work provides a possible way to convert waste HFC refrigerant mixture into high value-added chemicals and syngas.

Comparison study of supercritical water gasification for hydrogen production on a continuous flow versus a batch reactor

This study compares batch and continuous supercritical water gasification (SCWG) processes for green hydrogen production from biomass. It offers insights for optimizing processes, enhancing yields, quality, and energy efficiency, assessing scale-up feasibility, and supporting techno-economic analyses. Glucose, glycerol, and black liquor were SCWG-treated at 500 °C with K2CO3 catalyst in a self-built continuous-flow reactor (150 g/h) and a batch reactor (75 mL). Comparisons primarily focused on gas product yields. Batch reactors outperformed continuous-flow reactors in hydrogen (glucose: 1.53 to 0.9 mmol/g, glycerol: 7.22 to 1.14 mmol/g, black liquor: 2.88 to 1.74 mmol/g) and total gas yields due to differences in reaction time and heating behavior. Temperature effects on continuous operation (450–600 °C) were studied, with glycerol showing the highest hydrogen yield increase (from 1.21 to 4.30 mmol/g). The study discusses the applicability of both reactors for biomass SCWG processes and their implications for sustainable green hydrogen production from renewable feedstocks.

An experimental and thermodynamic equilibrium investigation of heavy metals transformation in supercritical water gasification of oily sludge

Supercritical water gasification (SCWG) is an advanced and highly efficient method for treating oily sludge. However, it is crucial to consider the transformation characteristics of heavy metals (HMs) during the SCWG process to prevent potential secondary pollution. This work studied the transformation and distribution characteristics of Cu, Cr and Zn after SCWG of oily sludge in a batch reactor at temperatures ranging from 550 to 700 °C. Additionally, thermodynamic equilibrium analysis was conducted to assess the distribution of HMs based on the minimization of Gibbs free energy. Experimental results indicated that higher temperatures led to the conversion of HMs into more stable forms, effectively immobilizing them within solid products. Furthermore, the addition of Na2CO3 enhanced this process and contributed to a reduction in HMs pollution in the effluent. Thermodynamic equilibrium results were consistent with our experimental data, indicating that the molar fraction of stable HMs forms followed the order: Cr > Cu > Zn. Besides, it is worth noting that Na2CO3 had a limited impact on the distribution of Cu and Cr. However, it played a significant role in inhibiting the formation of silicate Zn at lower temperatures, promoting the decomposition of ZnO*Al2O3 into unstable Zn. This may explain the higher presence of unstable Zn when Na2CO3 was introduced. In summary, this study offers valuable insights into the transformation characteristics of heavy metals and strategies for pollution control during SCWG of oily sludge.

A large-eddy simulation study of the transcritical mixing process in coaxial jet flow under supercritical condition

Supercritical water gasification (SCWG) is a promising clean energy technology for utilizing fossil fuels or organic solid waste with the challenge of delivering and dispersing liquid feedstock into the reactor. This paper investigates the transcritical mixing process in coaxial injections using large-eddy simulation. The results reveal that large temperature and density gradients exist between the inner injected water and the ambient fluid in the near field of transcritical water injection. Structure comparison demonstrates that the cross-section of the inner potential core shrinks more quickly in the non-premixing coaxial injection than the single injection, which can facilitate the dispersion and dilution of injected water during the heating process due to the shear mixing with the coaxial high-velocity outer injection. In addition, in the premixing coaxial injections, the substantial radial velocity fluctuation induced by the contracting cross-section leads to a much shorter inner potential core and more intensive heat and mass transfer. Swirling inlets can significantly strengthen the radial velocity fluctuation, thereby accelerating the temperature increase and species blending. Coaxial injections can effectively be used for dispersing feedstock in the SCWG process, particularly for high-viscosity materials.